The invention relates to a device for accurately controlled feeding of a fine-grained, free-running material from a storage silo into a reactor vessel, in particular alumina from a day's storage silo to a break in the crust on an aluminum fused salt electrolytic cell.
The production of aluminum by fused salt electrolysis of aluminum oxide involves the dissolution of the latter in a fluoride melt which is comprised for the greater part of cryolite. The cathodically deposited aluminum collects under the fluoride melt on the floor of the cell, the surface of the liquid aluminum itself forming the actual cathode. Dipping into the molten electrolyte from above are anodes which in conventional processes are made of amorphous carbon. At the carbon anodes oxygen is formed as a result of the electrolytic decomposition of the aluminum oxide; this oxygen reacts with the carbon of the anodes and forms CO.sub.2 and CO. The electrolytic process takes place in a temperature range of approx. 940.degree.-970.degree. C.
In the course of the electrolytic process the electrolyte becomes depleted of aluminum oxide. At a lower concentration of 1-2 wt% aluminum oxide in the electrolyte the anode effect occurs whereupon the voltage rises for example from 4-5 V to 30 V or more. Then at the latest the aluminum oxide concentration must be raised by adding fresh alumina to the cell.
Under normal operating conditions the cell is generally fed alumina at regular intervals even when no anode effect occurs. In addition, whenever the anode effect occurs, the alumina concentration must be raised by adding fresh aluminum oxide, which constitutes a servicing of the cell.
For many years now servicing the cell has included breaking the crust of solidified melt between the anodes and the sidewall of the cell and then adding aluminum oxide there. This method, which is still widely practiced today, is meeting with increasing criticism due to pollution of the air in the pot room and surrounding areas. The demand for hooding of the pots and treatment of the waste gases has increasingly become an obligatory requirement in recent years. Maximum capture of the pot gases is not possible with hooding if the classical method of feeding the cells at the long side between the anodes and the sidewall of the cells is maintained.
More recently, therefore, the aluminum producers have gone over increasingly to automatic feeding of the cell at its central longitudinal axis. The alumina is then fed either locally and continuously by so called point-feeding, or discontinuously along the whole longitudinal aixs of the cell. In both cases a storage bunker or silo for alumina is positioned on the cell. This applies to the inventors' recently proposed transverse feeding of electrolytic cells (DE-OS No. 27 31 908).
The known storage bunkers or alumina silos on the electrolytic cells are in the form of funnels or containers with funnel shaped lower parts. The contents of the silos on the cells usually suffice for one to two days feeding of the cell. The feeding of the alumina in the silo to a hole in the crust covering the molten electrolyte is, in the case of these known devices, carried out by means of an apportioning, sliding feed facility, a controlled feed screw and compressed air, a controlled feed screw and a piston compressor, a jacking and/or rotating valve device or by the sprinker method. Also known is the method of feeding alumina by gravity feeders and fluidizing channels.
Numerous, known controlled feeding devices sufffer the disadvantage that they are susceptible to breaking down and/or operate with insufficient precision.
It is therefore an object of the invention to develop a reliable and economically operable controlled feed device for supplying fine-grained, free-running particulate materials, said device being characterized by way of its high accuracy of feeding.